Tardigrades might be tiny, but they're mighty mysterious. Also known as water bears (or my personal favorite, moss piglets), the eight-legged micro-beasties can survive basically anything, including years of dehydration, massive doses of radiation and the vacuum of space.
How do they do it? On Tuesday, researchers published the most comprehensive genome sequence of a tardigrade species to date in the journal Nature Communications. The DNA of these tiny critters revealed a unique protein capable of protecting cells — even human ones — from radiation.
Last year, researchers at the University of North Carolina published a study claiming that tardigrades carried more "foreign" DNA — genes stolen from unrelated organisms by way of a process called horizontal gene transfer — than any other known animal. The scientists estimated that one-sixth of their tardigrade's DNA came from outside sources, which more than doubled the previous record of the rotifer.
That finding was later disputed by other research groups, and even the scientists involved have conceded that bacteria from their lab must have contaminated the UNC study. Tuesday's genome sequence from the University of Tokyo found that just around 1 percent of their tardigrade's DNA came from foreign sources, which is decidedly more average — that puts it on par with the genetic thievery of the nematode.
UNC's Bob Goldstein, co-author of the controversial 2015 study, expressed excitement about the new genome sequence — even though the results aren't quite in sync with his work on horizontal gene transfer. While he and his colleagues accept that their own tardigrade genome was likely contaminated, he points to other labs that have found tardigrades to carry some 4 or 5 percent foreign DNA (though these were upper limit estimates, with 1 or 2 percent being more likely). That would still make them unusually prolific in the world of horizontal gene transfer, suggesting they might have this practice to thank (at least in part) for their invincibility.
The University of Tokyo lab sequenced a different species of tardigrade: Ramazzottius varieornatus, one that has never had its genome sequenced before. But they argue that because this species is even more demonstrably resilient to extreme conditions than the species studied by Goldstein and other labs, the lack of horizontal gene transfer suggests that tardigrade superpowers are unrelated to any upticks in foreign DNA.
"I think we need a better assembled genome to help resolve that," Goldstein said when asked where he stands on the gene transfer question today. He added that the new paper looked "fantastic" and was "certainly the best assembled genome of any tardigrade to date."
In sequencing Ramazzottius varieornatus's genome, the researchers identified a protein unique to tardigrades — one never seen before that likely aids in the creature's death-defying feats.
The researchers believe the protein helps protect tardigrade DNA from the dangerous side effects of radiation exposure and repair any damage that does occur. When they manipulated human cells to make them produce the tardigrade protein, those cell cultures experienced some 40 percent less damage from X-ray exposure than normal cells.
Ingemar Jönsson, an evolutionary ecologist who studies tardigrades at Sweden's Kristianstad University but wasn't involved in the new study, told Nature that the findings were “highly interesting for medicine.”
For now, any human applications are totally speculative — though I welcome a future of genetically engineered moss-piglet-people populating the cosmos. But while it might be quite a while before we can steal tardigrade powers for ourselves, scientists are likely to find many more strange, protective proteins in water bears very soon.
"They survive a lot of different kinds of extremes," Goldstein said. "So there are probably a lot of kinds of protectants. It’s an exciting finding, and I expect we’ll find a lot more proteins like this."